The present invention is directed toward the technical field of time-of-flight (TOF) mass spectrometers and, more particularly, toward the improvement of duty cycle performance in TOF mass spectrometry.
In the publication, "The Ideal Mass Analyzer: Fact or Fiction?," International Journal of Mass Spectrometry and Ion Processes, Vol. 76, p125-237 (1987), which is incorporated herein, author Brunee discusses the birth of time-of-flight mass spectrometry during the 1950's. Time-of-flight mass spectrometry was at first predominantly used in the study of fast reactions. As time-of-flight mass spectrometers operate at very high scanning speed, data from spontaneous reactions can effectively be recorded at the very high rates of the explosions themselves, e.g. 10,000 mass spectra per second or more. Even though time-of-flight techniques were accordingly used in the past to study fast reactions such as explosions, other applications of this technique were neither widespread nor plentiful.
In fact, it was not until the late 1970's, when plasma desorption techniques were first applied to TOF mass spectrometry as described by Macfarlane, (see for example Brunee at page 151) that TOF mass spectrometry began to show promise in the analysis of high mass molecules. In particular, Macfarlane showed that high-mass molecules could be efficiently ionized and detected as well as low mass molecules.
As is now generally known, in TOF mass spectrometry, ionizing a sample provides a start impulse for the time measurement. The resulting ions are separated by their flight times and recorded using pulse counting techniques. The output of a multi-stop time-to-digital converter then provides a direct measure of the corresponding mass.
In ideal circumstances, all of the ions generated during TOF mass spectrometry operation are detected thereby avoiding detection losses due to scanning from mass to mass as in the case of quad and sector instruments. Conventionally, the plasma desorption TOF technique is combined with a liquid chromatograph for the identification of high molecular weight compounds as well as for elemental trace analysis of solids. Theoretically, there is no detection limit with respect to mass range analysis. Mass separation is solely dependent on flight time, while scanning and recording speeds depend solely on cycle and flight time.
This variant of time-of-flight mass spectrometry has however generally shown only limited sensitivity with small sample amounts. Furthermore, ionization efficiency drops considerably with increasing molecular weight. As heavy molecules need a higher density of energy for their ablation than available by plasma desorption time-of-flight techniques, mass range and resolution have been limited.
Attempts to increase mass resolution have met with little success. In the publication, "The Renaissance of Time-Of-Flight Mass Spectrometry," International Journal of Mass Spectrometry and Ion Processes, Vol. 99, pages 1-39 (1990), which is hereby expressly incorporated herein, authors Price and Milnes illustrate the many methods that have been attempted to improve the resolution. A common method of increasing the mass resolution, according to Price and Milnes, is reducing the velocity spread of the ions. Often, this is achieved by reflectron techniques under application of decelerating and reflecting fields. In an attempt to achieve mass-independent space and energy focusing, another method proposed is dynamic post-source acceleration. Kinsel and Johnston suggested using post-source pulse focussing as a method to improve resolution in linear time-of-flight mass spectrometry, while Muga applied the principle of velocity compaction to improve resolution in this work, Analytical Instruments, vol. 16, page 31 (1987).
The prior art methods referenced above are limited in their respective practical applications. Generally, mass resolution is gained by either deflecting, reflecting, or controlling the velocity of the particle spread. Conventional time-of-flight mass spectrometry apparatus further all employ similar means for stimulus. For example, single pulse ion sources establish time resolution in the ion transport to the detector. Conventional TOF spectrometers avoid overlap in flight times at the detector by making the scan repeat time (cycle time) at least as long as the flight time of the heaviest mass ion. This long cycle time coupled with the pulsed ion production time leads to a very small duty cycle and consequently very limited ion abundance sensitivity.
Furthermore, time-of-flight mass spectrometers are currently not fully compatible with all kinds of available ionizing sources. For example, a single chemical ionization source cannot be pulsed sufficiently rapidly for satisfactory resolution in normal operation of a TOF spectrometer. According to another ionization alternative, an electrostatic energy analyzer can be introduced between the ion source and a linear time-of-flight mass analyzer (TOFMA). This however improves resolution at the price of sensitivity.
Time-of-flight instruments are used in fields other than analytical and physical chemistry. Large research instruments have been built for the identification of high-energy particles in nuclear physics experiments. These instruments have also incorporated magnetic deflection. Time-of-flight instruments as applied to space-science studies are further especially useful in the analysis of solid particles.
Conventional time-of-flight mass spectrometry uses a single pulsed ion source to establish time resolution from the ion transport to the detector. The best duty cycle achievable using such systems is significantly less than 50%. Simply put, only a small fraction the sample is available for analysis. In situations in which a limited amount of the sample material is available, insufficient data is thus gathered adequately to study the ions.
What is accordingly needed is a time-of-flight mass spectrometry approach which enables maximum sensitivity, i.e. optimal use of the sample, prior to analysis.